Headache and high heart rate after using Minoxidil?

What your describing can be very scary.  I recommend you discontinue the Minoxidil and visit a physician if you have other incident. 

it could be a side effect of either the minoxidil or the caffeine usage. I would recommend being seen by the physician who prescribed the minoxidil, and if you are having persistent tachycardia to be seen at an urgent care clinic or emergency room. 

In my opinion the issues could be related to topical Minoxidil side effects. Please stop the medication and contact that doctor inmediately who prescribed the medication.

Minoxidil can lower blood pressure and raise heart rate in some people. Usually they are temporary.  Urgent in person medical attention is necessary and appropriate (rather than online) if the heart rate elevations persist. Often the emergency department is the most appropriate place for evaluation of persistent tacchycardias. Minoxidil must be respected, even when applied topically. Side effects on the heart are possible - rarely abnormal heart rhythms can develop that need medical intervention. Overall, serious side effects are very rare with topical minoxidil. But like any medication, one needs to get appropriate medical attention if worrisome side effects develop.

A headache and high heart rate are known side effects of minoxidil which is being absorbed through your skin and causing hypotension